Temperature monitoring apparatus and techniques using magnetic resonance are known in the art.
Temperature measurements based on Nuclear Quadrupole Resonance (NQR) are described in the following references:
Yu. N. Bubkov, R. Sh. Lotfullin, R. V. Majera, and A. A. Buguslavskii--"Pulsed NQR thermometer for temperatures up to 870 degrees K.", Izvestiya Akademii Nank SSSR, Seriya F. Z. Cheskaya, Vol. 39, No. 12, 1975, pp. 2655-2660;
D. B. Utton, J. Vanier--"Thermometry by Nuclear Quadrupole Resonance", Instrumentation Technology, December 1976, pp. 47-52;
A. Ohte and H. Iwaaka--"A new nuclear quadrupole resonance standard thermometer" Temperature, American Institute of Physics, 1982, pp 1173-1180.
The essential feature of the NQR technique is measurement of temperature-dependent NQR frequencies which are an intrinsic property of the material used to construct an NQR sensor.
In laboratory contexts wherein various types of magnetic resonance were used to measure parameters of material structure, such as the lattice magnetic field, the temperature dependence of nuclear magnetic resonance (NMR) frequencies has been investigated. Reference is made in this context to S. Ogawa and S. Morimoto--"Nuclear Magnetic Resonance of Fe.sup.57 in Various Ferromagnetic Oxides", Journal of the Physical Society of Japan, Vol. 17, No. 4, April 1962, pp 654-659. This reference shows, inter alia, at Tables I(a), I(b) and II, the relationship between internal magnetic field and temperature.
Thermometry systems in conventional use in clinical hyperthermia include probes employing thermocouples and thermistors having non-metallic output leads, so as not to interfere with the applied electromagnetic fields. Where multiple location temperature sensing is required, this is conventionally accomplished by moving the probe from location to location or by employing a plurality of such probes.
The state of the art in the use of thermometry systems in clinical hyperthermia is described in G. M. Samaras, A. Y. Cheung, "Microwave hyperthermia for cancer therapy", CRC Critical Reviews in Bioengineering, February, 1981, pp. 123-184. Some of the techniques described in the foregoing article are based on temperature dependence of optical parameters of given substances, such as GaAs, and liquid crystals. The advantages of such techniques are that output communication is provided by optical fiber leads, which do not interfere with the applied electromagnetic field. A multiple GaAs sensor is described in V. A. Vaguine, D. A. Christensen, J. H. Lindley, T. E. Walston, "Multiple sensor optical thermometry system for application in clinical hyperthermia", IEEE Transactions on Biomedical Engineering, Vol. BME-31, No. 1, January, 1984, pp 168-172.
The use of conventional thermometry systems in clinical hyperthermia provides relatively poor reproducibility over time and relatively poor spatial resolution, in view of the fact that the probes are inserted prior to each treatment. The use of conventional probes naturally involves very significant patient discomfort and is time intensive and costly.
A number of proposed methods of noninvasive thermometry are described in T. C. Cetas, "Will thermometric tomography become practical for hyperthermia treatment monitoring?", Cancer Research (SUPPL.) 44, October 1984, 4805s-4808s. The methods proposed therein all involve unsatisfactory compromises between reading rate, temperature sensitivity, spatial discrimination, stability and reproducibility.
It has been proposed to employ non-zero external static magnetic field NMR for directly measuring the temperature of living tissue in vivo in a clinical hyperthermia context. Such proposals are based on temperature dependence of NMR relaxation times and are described in the following publications:
D. L. Parker, "Application of NMR imaging in hyperthermia: an evaluation of the potential for localized tissue heating and noninvasive temperature monitoring", IEEE Transactions on Biomedical Engineering, Vol. BME-31, No. 1, January 1984, pp 161-167; and
S. Peterson, J. Saaf, M. Bolmsjo, B. Persson, "Temperature mapping using an Ultra Lowfield MR scanner", Sixth Annual Meeting of Society of Magnetic Resonance in Medicine, Aug. 17-21, 1987, New York City, U.S.A.
If realized, the technique described in the preceding two publications would provide non-invasive direct measurement of temperature of living tissue. However, it would require expensive instrumentation which would have to be compatible with clinical hyperthermia apparatus. Moreover, the relaxation parameters are not uniform in different subjects. Therefore, in order to establish an absolute temperature output, a complicated calibration procedure would be required.